Ezine

Published: Apr 14, 2016

Author: Ryan De Vooght-Johnson

Channels: Gas Chromatography

The other ‘bad cholesterol’

Cholesterol is often demonised as the cause of health conditions, including heart disease and high blood pressure. However, this lipid molecule is also an essential part of cell membranes and a vital precursor to hormones, including cortisol, which helps maintain blood sugar levels, and the sex hormones testosterone and oestrogen.

It’s true that too much LDL cholesterol (‘bad cholesterol’, found in red meat and high fat dairy products) is bad for us, but there is a lesser known threat from oxidised cholesterol. In the presence of heat, light, moisture and oxygen, cholesterol oxides can form. These chemicals are very similar to cholesterol but contain an extra functional group, such as an OH (hydroxyl). More than 60 cholesterol oxides have been identified; some occur naturally but they are also present in a range of foods, especially those that are processed.

Although these compounds are similar in structure to cholesterol, they may be more harmful to health. Cholesterol oxides have been linked not only to heart disease but also cancer, including skin, breast, prostate and colon cancer.

While science is aware of the dangers of cholesterol oxides, detecting them remains a challenge. Gas chromatography (GC) can detect the molecules, but lacks resolution. There are also problems with sample preparation, which comprises multiple steps and can fundamentally change compounds, affecting analysis. Lixiviation, used to extract lipids, and silylation, for deriving cholesterol oxides, are both time consuming and often fail to produce clean extracts.

Enhanced extraction

To speed things up, accelerated solvent extraction (ASE) can be used for sample preparation. As its name suggests, ASE massively reduces the time it takes to perform extraction by using high temperatures and pressures, while using solvents similar to standard liquid extraction techniques. ASE reduces extraction time to under 15 minutes for samples under 30 grams and with a small volume of solvent. This method clearly has significant potential for studying cholesterol oxides in food, but has so far only been applied to powdered samples.

In a study recently published in the Journal of Science and Food and Agriculture, researchers tested the technique on prepared or ‘ready-to-eat’ foods. These foods have become very popular to meet growing demands for convenience, but may represent an underestimated health risk due to their cholesterol content.

Analytical chemists from the Complutense University of Madrid used ASE (with a mixture of petroleum ether and chloroform as the solvent) in combination with GC-MS, an established technique for cholesterol oxide analysis. They tested the technique on samples of minced beef, cooked ham, Serrano ham, soft cheese and smoked salmon, bought from a supermarket in Madrid.

When they compared the ASE method with the current standard of lixiviation, ASE was found to be much more effective. It reduced analysis time by a factor of 15, cut solvent volume in half and reduced the need for chlorinated solvents.

The safest sample: smoked salmon

Using mass spectral libraries, the researchers identified a range of cholesterol oxide derivatives in the food samples, including 7β-hydroxycholesterol, beta- and alpha-epoxide cholesterols, 7-ketocholesterol, and 25-hydroxyl cholesterol. Minced beef contained the highest concentrations, which the authors attribute to its high contact surface for oxidation, while smoked salmon contained the least cholesterol oxides.

The processing of these foods, which includes handling, cutting and packaging, increases the risk of bacterial contamination. To prevent food poisoning and increase shelf life, electron beam irradiation can be used to kill microbes. The researchers wanted to know how this process affects cholesterol oxide levels, so compared concentrations between irradiated and non-irradiated food samples.

The results showed that cholesterol oxide content increased following irradiation. As well as irradiation dose, the water and fat content of the food was linked to cholesterol oxide concentration, although the authors say more work is necessary to fully understand these connections.

Overall, this study shows that oxidised cholesterol products are present in a range of prepared foods, and may be linked to fat content and the use of e-beam irradiation, which will be important information for manufacturers, food scientists and regulatory authorities. It could also make future work more efficient by encouraging use of the ASE method for preparing samples.